The state-of-the-art electronic device comprises a number of the miniaturized electronic components per unit volume. These electronic components are highly efficient and capable of high performance, thereby resulting in massive generation of heat in the course of their operation. In light of design variation of the electronic components, the heat flux distribution on the surface of the electronic components is apt to be uneven, so as to form the so-called “hot spot” on the surface of the electronic components. Such a locally over-heating phenomenon is detrimental to reliability and longevity of a highly-sophisticated electronic device, such as notebook computer.
In order to prepare for advent of electronic products of new generation, a number of passive cooling elements have been introduced into the market place. These passive cooling elements have the same working principle. As shown in FIG. 1, a vacuum chamber 1 is provided in the surface of an interior thereof with a wick structure 2. Meanwhile, the vacuum chamber 1 is provided with a working fluid, which is distributed on the wick structure 2 by virtue of capillarity. As the chamber 1 comes in contact with a heat source, the working fluid is heated by the heat source to evaporate to remain in the form of vapor. When the working fluid vapor comes in contact with a cooler portion of the chamber, the working fluid vapor condenses to remain in the form of liquid. The liquid is then guided to the wick structure containing lesser amount of liquid by virtue of capillary force brought about by the wick structure. As a result, a subsequent cycle of evaporation and condensation is effected such that the heat is transferred from a hotter region to a colder region, with a minute change in temperature. It is therefore readily apparent that the wick structure is critical to the design of the passive elements described above, and that the wick structure serves as a passage of the liquid as well as a driving force of the liquid. As a result, a liquid/vapor dual phase cycle of the working fluid takes place smoothly in the vacuum chamber. However, the wick structure is also an obstacle to heat transfer due to its low thermal conductivity. In another words, the liquid which is attracted to the wick structure would fail to vaporize as expected, thereby resulting in a poor heat dissipation or heat distribution.
As shown in FIG. 2, the Taiwan Patent Serial No.89210557 discloses a flat heat pipe comprising a vacuum chamber 3 in which an appropriate amount of a working fluid is contained. The vacuum chamber 3 is provided with a plurality of wick structures 4, which are connected with an upper wall and a lower wall of the chamber 3 for enhancing the structural strength of the flat heat pipe, and for increasing the number and the surface area of the wick structure. In spite of the high-density distribution of the wick structure to promote the flow of the condensate, the wick structure is in fact an obstacle to heat transfer due to the fact that the wick structure is relatively low in thermal conductivity. This prior art flat heat pipe is ineffective in heat transfer of the electronic components, especially those electronic components which generate heat unevenly to form hot spots.
The Taiwan Patent Serial Number 86115415 discloses a cooling device comprising a chamber 5 in which an appropriate amount of working fluid is contained, as illustrated in FIG. 3. The chamber 5 is provided with a number of cooling fins 6, fluid conduction pillars 7, and wick structures 8. The fluid conduction pillars 7 serve a dual-purpose of support and fluid conduction effect. The wick structures 8 are intended to increase the contact area between liquid and heat source, and to bring about the liquid conduction effect of condensate. The fluid conduction pillars 7 have no specific effect on heat transfer and hot spot. In another words, this prior art cooling device is ineffective at best.
The Taiwan Patent Serial No.88210055 discloses a cooling device comprising a chamber 9, an upper plate 10, and a lower plate 12, as shown in FIG. 4. The upper plate 10 is provided with a number of projections 11, whereas the lower plate 12 is provided with a wick structure 13 which comes in contact with the projections 11. The reflux and the conduction of condensate are attained by the wick structure 13. A support effect is jointly brought about by the wick structure 13 and the projections. In light of the wick structure 13 being relatively low in thermal conductivity, the wick structure 13 is in fact an obstacle to heat transfer. Both the wick structure 13 and the projections 11 are ineffective in terms of heat dissipation and uniform temperature distribution. In particular, this prior art cooling device is inefficient to deal with the problem of hot spot of electronic components.